It is possible to transmit films and audiovisual programs in digital form via broadcasting networks of the microwave, herzian, cable, satellite type, etc. or via telecommunication networks of the DSL (digital subscriber line) type or BLR (local radio loop) type or via DAB (digital audio broadcasting) networks, etc. Moreover, to avoid pirating of works broadcast in this manner, the latter are frequently encrypted or scrambled by various well known means.
WO 00/60846 discloses distribution of video on demand via a system of distributed servers. Protection of the digital video content is based on a system of encryption based on digital keys. That permits an encryption and decryption of streams in conformity with the MPEG-TS (MPET transport stream) norm, simplified in that an analysis of the MPEG stream is made in advance to optimize the encryption as a function of the structure of the stream. In fact, the calculation time is greatly reduced by virtue of a selective and optimum encryption of the MPEG-TS stream. Analysis of the stream consists principally of marking the TS packets containing information essential for a correct decoding of the video in the MPEG sense in such a manner that in a second encryption phase only the useful data of the marked TS packets is encrypted. Thus, the essential information is not available for a non-authorized decoding and the stream can therefore not be displayed while preserving a reasonable calculation time for the encryption. However, that solution does not allow a resolution of the security problem because the protection of the video stream is based on an encryption system based on digital keys but for which the video stream transmitted to receiving equipment contains in encrypted form all the information initially present in the original video stream. Thus, the video stream received by the client equipment already contains information initially present in the non-encrypted video stream even if they are stored in a form that does not permit their immediate use. An ill-intentioned user, even if disconnected from the network, can process the encrypted video stream in such a manner as to return it to a form that can be used by a classic video decoder and can thus circumvent the protection system.